FIG. 1 is a plot illustrating waveforms associated with operation of a PFC boost converter operating in Borderline Conduction Mode (BCM). An example PFC boost converter circuit is shown in FIG. 2.
The plot shows waveforms for output voltage Vout, input voltage Vin and inductor current Il. When using a microcontroller to control PFC in a boost converter operating in BCM, the boost switch conduction time (Ton) is maintained constant over each half cycle of the input sinusoidal voltage. The peak inductor current Il for each switching cycle is proportional to the input voltage Vin which is nearly constant during Ton (Il peak=Vin×Ton/L). Since the average value of the triangular Il waveform is half its peak value, the average current drawn is also proportional to the input voltage Vin. This implies that Vout is composed of a continuous voltage plus a rectified sinusoidal component of the same frequency as the rectified input voltage Vin. Because of the rectified sinusoidal component of Vout, to stabilize the Vout regulation loop of the converter, measurement of Vout must be done each cycle at the same position of the main supply period. This can be accomplished by detecting a reference point of the input voltage period. Generally, the Vin zero crossing point is taken as the most obvious reference point of the main supply period.
FIG. 2 is a conventional PFC boost converter 200 controlled by a microcontroller 204. FIG. 3 illustrates waveforms associated with the operation of converter 200. Converter 200 can include rectifier 202 (D1-D4), microcontroller 204, energy storage inductor 206 (L), boost switch 216 (e.g., MOSFET), divider circuits 218 (R1, R2), 220 (R3, R4), 224 (R5, R6), capacitor 222 (C) and diode 226 (D5). Microcontroller 204 can include comparator 208, PWM module 210, central processing unit (CPU) 221 and ADC 214.
In the configuration shown, full-bridge rectifier 202 (a diode bridge) rectifies voltage, Vac, to provide rectified input voltage Vin. In an “on” state, switch 216 is closed by PWM module 210 for switch conduction time Ton, resulting in an increase of Il in energy storage device 206 (e.g., an inductor) due to Vin. In an “off” state, switch 216 is opened by PWM module 210 and the only path offered to Il is through diode 226, capacitor 222 and the load. This results in transferring energy accumulated in inductor 206 during the on state into capacitor 222. FIG. 3 illustrates the triangular waveform for current Il that is generated by the on state and off state of switch 216, which is commanded by PWM module 210
Current in a secondary coil coupled to energy storage inductor 206 is taken from current divider 220 and supplied to high and reverse voltage protected input IZCD to PWM module 210, where it is used to detect the end of Il decrease time to initiate a new PWM cycle. Vout is an analog value taken from voltage divider 224 and supplied as feedback (FB) to the voltage regulation loop of converter 200. At each zero crossing of Vin, ADC 214 converts FB into a digital value, which is provided as input to CPU 212. CPU 212 computes via a proportional-integral-derivative (PID) algorithm an updated compare value (cmp) to PWM module 210 based on FB. PWM module 210 updates Ton based on cmp to adjust Vout.
In some implementations, an external specialized component (e.g., an optocoupler external to microcontroller 204) can be used to detect the zero crossing point of Vin. In converter 200, Vin is taken from voltage divider 218 and supplied as input VZCD into microcontroller 204. The VZCD input to microcontroller 204 is configured as an input of internal comparator 208, or an input channel of internal ADC 214. FIG. 3 shows that VZCD is about zero at the zero crossing point of Vin, causing the output of comparator 208 to provide a command to ADC 214 to sample Vout.
Using comparator 208 or ADC 214 in free running mode (continuous conversion) in microcontroller 204 for detecting main supply zero crossings is costly to implement and can limit the application field that microcontroller 204 can cover. For example, if microcontroller 204 is used to drive an electrical motor with its power supply, the bandwidth of ADC 214 needed for Vin zero crossing detection can make the motor control unfeasible or limited, and the use of a comparator for Vin zero crossing detection can make the motor control costly because of the use of an additional external comparator due to a lack of internal comparator resources.